The Smith predictor (SP) provides a nice controller structure for the deadtime compensation, and it has received considerable attention over past 40 years. Initially, the implementation difficulties encountered in the analog era (60s–80s) had led to little use in process industries. In the following years (80s-), the microprocessor-based implementation makes the SP a new feature in many process control computers as well as single station controllers. However, the modeling requirement, non-trivial tuning, and unfamiliarity prevent wide-spread applications. A typical scenario is that, in order to maintain robust stability, the associated PID (proportion integration differential) controller is detuned to such a degree that the control performance is no better than that of a simple PID controller, and this is especially true for systems with small deadtime to time constant ratio (D/τ). Despite the clear advantage for deadtime dominant processes, the Smith predictor again finds the limited applications throughout process industries. On the other hand, it has been found that the PID controllers remain strong in the digital age.
As mentioned above, the three application difficulties associated with the SP are the modeling, tuning, and familiarity. For the modeling requirement, it can easily be overcome by using relay feedback tests which also becomes a standard feature in many process control computers. Based on the shape information, the model structure as well as model parameters can be identified in a single relay feedback test. The remaining tuning and familiarity problems of the SP are treated simultaneously in the present invention.
It has been proved that the performance of the SP (as shown in FIG. 1(b)) is inferior to the PID controller (as shown in FIG. 1(a)) for first order plus deadtime systems (FOPDT) with the deadtime to time constant ratio (D/τ) less than 0.2. The PID controller shown in FIG. 1(a) includes a feedback controller K and a process G. In which, the feedback controller K is used for generating a first function u in response to the difference between a set point yset and a closed-loop transfer function y, and the process G is used for generating the closed-loop transfer function y in response to the sum of the first function u and a load variable L of the system. The SP shown in FIG. 1(b) includes the feedback controller K, the process G and a deadtime compensator 13. In which, the feedback controller K is used for generating the first function u in response to the difference between the set point yset and the closed-loop transfer function y, the process G is used for generating the closed-loop transfer function y in response to the sum of the first function u and the load variable L of the system, and the deadtime compensator 13 is coupled to the feedback controller K for generating a third function in response to the first function u, wherein the feedback function is the difference between the second function and the third function. Moreover, for the PID controller, the advantage of the SP can only be seen for FOPDT systems with D/τ greater than 10. It is troublesome because how a model-based approach, with correct controller structure, would fail to achieve improved performance. The reason is obvious that the SP is significantly detuned to achieve a certain degree of robustness (or the PID settings were tightened to obtain better performance). In other words, if a model-based controller is loosely tuned, the performance can not be better than a PID controller. This scenario is often encountered in practice, especially when the users are not familiar with the controller and its tradeoff between the robustness and performance.
From the above descriptions, it is known that how to develop a new method and apparatus for PID controller with the advantage of ensuring a certain degree of robustness has become a major problem waited to be solved. In order to overcome the drawbacks in the prior art, a method and apparatus for PID controller with an adjustable deadtime compensation is provided. The present invention not only solves the problems described above, but also can be implemented in current process control computers with virtually no extra hardware cost. Thus, the invention has the utility for the industry.